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Canada's top court clarifies the law on causation for negligence claims

In June, the Supreme Court of Canada released its ruling in Clements v. Clements, a decision in which the court provided some guidance on proving causation in cases of negligence. In particular, the court set out the circumstances in which the exceptional “material contribution” test is to be utilized.

Under the law of negligence, the plaintiff must prove that the defendant’s negligent conduct caused the injuries suffered. Causation can be proved by applying either one of two tests: the “but‑for” test or the material contribution test. Under the but‑for test, the plaintiff must prove that, but for the defendant’s negligent conduct, the plaintiff would not have suffered his or her injuries. Under the material contribution test, however, the plaintiff need only show that the negligent act materially contributed to the risk that the injury that did occur would occur. While lower courts in Canada understood that the material contribution test should only be applied in “special circumstances”, until its decision in Clements, the Supreme Court of Canada had provided very little in terms of concrete guidelines as to when the material contribution test should be applied.

The case of Clements involved a single vehicle accident in which the defendant was driving a motorcycle with his wife, the plaintiff, riding behind on the passenger seat. The couple were riding in wet weather and the bike was carrying approximately 100 pounds over capacity. Unbeknownst to the defendant, a nail had punctured the rear tire of the motorcycle. When the defendant accelerated over the speed limit in order to pass a car, the nail fell out and the rear tire began to deflate. This caused the motorcycle to develop a “weave instability”, from which the defendant was unable to regain control. The motorcycle subsequently crashed, causing the plaintiff to suffer a severe traumatic brain injury.

The plaintiff claimed that her injuries were caused by her husband’s negligence. In particular, she argued that, in the event that she could not prove causation under the conventional “but‑for” test, then “special circumstances” warranted the application of the material contribution test in her favour.

At the British Columbia Supreme Court, the defendant was found to have breached the standard of care by excessively speeding and overloading the motorcycle. In regards to the element of causation, however, the trial judge found that the weave instability was not caused by the motorcycle’s excessive speed or load but rather by the rapid deflation of the rear tire at normal highway speeds. Furthermore, the judge was unable to determine based on the available science whether, but for the excessive speed and overloading, the defendant would have been able to recover safely from the weave instability. In the end, however, the trial judge agreed with the plaintiff that special circumstances existed to justify the application of the material contribution test and, under that test, found that the defendant’s breaches had increased the risk that the plaintiff would be injured if the motorcycle became unstable. The defendant was, thus, held fully liable.

This decision was reversed, and the case was dismissed, by the British Columbia Court of Appeal. In that court’s view, the trial judge had inappropriately applied the material contribution test, a policy‑driven test that, in the court’s opinion, should be resorted to only in exceptional circumstances where causation cannot be logically proven and fairness and justice require its application.

At the Supreme Court of Canada, Chief Justice McLachlin clarified the law on the circumstances in which the material contribution test is to be utilized, an elucidation of the law that was endorsed by all nine judges. In the Chief Justice’s view, the but‑for test will apply in the majority of cases, although certain exceptional circumstances will justify the use of the material contribution test. In particular, McLachlin C.J.C. held that, in order for the material contribution test to apply, the following factors need to be present:

The case must involve multiple tortfeasors, each and everyone of which must have been found to have engaged in some negligent conduct;

The plaintiff must show that the negligent conduct of at least one of the tortfeasors factually caused the plaintiff’s injuries; and

The plaintiff, through no fault of his or her own, must be unable to show which one of the tortfeasors was in fact the necessary cause of the injury because each tortfeasor is able to point to another tortfeasor as the possible but for cause of the injury.

In light of these factors, the Chief Justice found that the circumstances of the Clements case did not warrant the application of the material contribution test, and that the trial judge had erred in applying the test.

The Chief Justice also explained that the but‑for causation test must be applied in a “robust common sense fashion”. In other words, the plaintiff is not required to show with scientific precision the degree of contribution that the defendant’s negligence made to the injury. In this regard, the trial judge had also erred by insisting that the plaintiff prove but-for causation with scientific reconstruction evidence.

In the end, the 7‑2 majority of the Supreme Court ordered that a new trial take place, on the basis that the parties did not receive a trial based on correct legal principles.

As a result of the Supreme Court of Canada’s decision in Clements, it is now clear that the material contribution test is only applicable in a very limited set of circumstances. Echoing the comments made by the British Columbia Court of Appeal, the Supreme Court of Canada has explained that the material contribution test is a policy‑driven rule, and that its application is “necessarily rare” and justified only where it is required by fairness and justice. In the vast majority of negligence cases, plaintiffs will need to prove causation on the higher threshold required by the but‑for test.